This application incorporates by reference Taiwanese application Serial No. 89211927, Filed Jul. 11, 2000.
1. Field of the Invention
The invention relates in general to a cell voltage balancer, and more particularly to a cell voltage balancer which is capable of balancing the voltage of numerous cells which are connected in series.
2. Description of the Related Art
Unbalance charging usually occurs in cells connected in series. Cells connected in series have equal current flowing through but may not be equally charged due to various cell voltages and cell capacities of the cells. Consequently, some cells are overcharged while some are not fully charged.
The lifetime of overcharged cells could be shortened as a result of raised temperature. Irreversible chemical reaction may also occur, which reduces the performance of the cells and causes permanent damage of the cells.
Therefore, the application of a cell voltage balancer is important. FIG. 1, is a schematic diagram showing charging cells connected in series with the aid of a conventional cell voltage balancer. Four cells in charging are taken as an example. The conventional cell voltage balancer 100 includes resistances R1, R2, R3, R4, switches S1,S2,S3, and S4. Cells B1,B2,B3 and B4 are charged by charger 101. Cell B1 is connected in parallel with resistance R1 and switch S1 connected serially; cell B2 is connected in parallel with resistance R2 and switch S2 connected serially; cell B3 is connected in parallel with resistance R3 and switch S3 connected serially; and cell B4 is connected in parallel with resistance R4 and switch S4 connected serially. The voltage V1, V2, V3, and V4 of the respective cells B1, B2, B3 and B4 are inputted into the control signal generator 102 for controlling the switches S1, S2, S3 and S4.
While in charging mode, The charger 101 can, for example, provide a constant current I to charge the cells. An average voltage V of the cells B1, B2, B3 and B4 is obtained after the voltages V1, V2, V3 and V4 are received by the control signal generator 102. The voltages V1, V2 V3 and V4 of each cell are then compared with the average voltage V. If the voltage of a cell is higher than the average voltage V, the corresponding switch is set on.
For example, while the voltage V1 of the cell B1 is higher than the average voltage V, and the voltages of the cells B2, B3 and B4 are all lower than the average voltage V, the control signal generator 102 sets the switch S1 on and the switches S2, S3 and S4 off. Meanwhile, the current I will be separated in two directions. The first direction is still to charge cell B1. The second direction is through R1 and S1. The charging current of B1 is less then the others, so the voltage rise of B1 will less then others.
When the charger 101 stops charging the cells B1, B2, B3 and B4 and these cells are not balanced in voltage, the control signal generator 102 can still receive voltages, V1, V2, V3 and V4 and set the corresponding switch on or off in order to balance the voltage of the cells.
However, the conventional method and mechanism stated above has the following disadvantages:
1. the resistance R1 consumes a part of the electrical energy and therefore the efficiency of the charger is reduced and the energy dissipated in resistance R1 is wasted; and
2. the heat produced by the resistance R1 raises the surrounding temperature, which reduces the performance of the cells.
To reduce the disadvantage of energy consumption of the cell voltage balancer as shown in FIG. 1, the discharging resistance is replaced by a transformer. FIG. 2A is a circuit block diagram showing another conventional cell voltage balancer applicable to charging serially connected cells. And FIG. 2B illustrates the operation of the circuit as shown in FIG. 2A. The transformer 202 includes a primary winding 204 and a secondary winding 206 and the transformer 208 includes a primary winding 210 and a secondary winding 212. The primary windings 204, 206 are serially connected with the switches S1, S2, respectively. The secondary windings 206,212 are serially connected with the diodes D1, D2, respectively. The transformers 202, 208, switches S1, S2, and diodes D1, D2 together form a conventional cell voltage balancer 214.
Cell B1 and the serially connected primary winding 204 and switch S1 are connected in parallel. Cell B2 and the serially connected primary winding 210 and switch S2 are connected in parallel. The secondary winding 206 and the diode D1 connect to the secondary winding 212 and the diode D2 in parallel. The turns of the secondary windings 206, 212 are at least two times of that of the primary windings 204, 210. The following example is taken for illustration, assuming that the of the secondary windings 206, 212 is two times of the primary windings 204, 210 and the voltage of cell B1 is larger than that of cell B2.
As shown in FIG. 2B, when the voltage of cell B1 is larger than the average voltage of cell B1 and cell B2, the switch S1 is turned on and cell B1 discharges. Meanwhile, current I1 flows through the primary winding 204. Because the voltage across the primary winding 204 is equal to the voltage of cell B1, the voltage across the secondary winding 206 is two times of that of the primary winding 204; that is two times of the voltage of the cell B1. Consequently, the voltage of the node N1 is larger than the sum of the voltage of cell B1 and the voltage of cell B2. So, the induced current 12 of the secondary winding 206 flows through diode D1 and cell B1 and flows toward cell B2 to charge cell B2. Consequently, the voltage of cell B1 is decreased to effectively avoid overcharge of cell B1. In addition, the energy released by cell B1 transfers to cell B2.
Similarly, when the charger 200 stops charging cell B1 and cell B2, the voltage unbalance of cells B1 and B2 may occurs and the cell voltage balancer 214 could still function to balance the voltage of the cells.
FIG. 3 shows the conventional cell voltage balancer of FIG. 2A which charges n serially connected cells. In order to recover energy to the n serially connected cells, the voltage of the secondary winding must be n times of that of the primary winding. Thus, the turns of the secondary windings 3041, 3042. . . , 304n of the transformers 3021, 3022. . . , 302n should be more than n times of that of the primary windings 3061, 3062. . . 306n. Although the conventional cell voltage balancer is capable of recovering the energy of high voltage cells to others, the design of the cell voltage balancer therefor becomes much more complex and the size of the transformer is thus enlarged due to the various turns of the transformer while various number of cells are used.
It is therefore an object of the invention to provide a cell voltage balancer which effective recovers energy with the aid of a transformer. The transformer of the invention is provided in the form of a module so that the turns of the transformer will not be a function of the number of series connected cells. The turns of each transform is fixed so that the circuit design is simplified and the size of the transformer is reduced.
It is another object of the invention to provide a cell voltage balancer for balancing the voltage of the first cell and the voltage of the second cell. The cell voltage balancer includes the first input terminal, the second input terminal and the third input terminal. The cell voltage balancer includes a transformer, the first switch, the second switch, the first diode and the second diode. The transformer includes a primary winding and a secondary winding. The first switch and the primary winding are serially connected between the first input terminal and the second input terminal. The second switch and the secondary winding are serially connected between the second input terminal and the third input terminal. The first switch and the second switch are switched on and off alternatively. The first diode and the first switch are connected in parallel and the second diode and the second switch are connected in parallel. While the first switch is on and the second switch is off, the primary winding stores energy of the first cell in the transformer and while the first switch is off and the second switch is on, the secondary winding recovers energy stored in the transformer into the second cell.
It is therefore a further object of the invention to provide a cell voltage balancing system for balancing the voltage of a number of cells. The cell voltage balancing system includes a number of cell voltage balancers. Each of the cell voltage balancers includes the first input terminal, the second input terminal and the third input terminal. Each of the cell voltage balancers is connected to two adjacent cells, the first cell and the second cell, of the cells. The second input terminal is connected to the common node of the first cell and the second cell. Each of the cell voltage balancer includes a transformer, the first switch, the second switch, the first diode and the second diode. The transformer includes a primary winding and a secondary winding. The first switch and the primary winding are serially connected between the first input terminal and the second input terminal. The second switch and the secondary winding are serially connected between the second input terminal and the third input terminal. The first switch and the second switch are switched on and off alternatively. The first diode and the first switch are connected in parallel and the second diode and the second switch are connected in parallel. While the first switch is on and the second switch is off, the primary winding stores energy of the first cell in the transformer and while the first switch is off and the second switch is on, the secondary winding recovers energy stored in the transformer into the second cell.